1. Field of the Invention
This invention concerns vehicle tire pressure monitors for setting in the rim of a wheel equipped with a tubeless tire, in which a switch is operated when the tire pressure falls too low, changing the character of an electrical circuit contained in the unit so that the changed condition of the switch may be detected by a sensor fixed on the vehicle so that it does not rotate with the wheel and connected in an alarm circuit.
2. Description of the Prior Art
In a known pneumatic switch unit of the above-described type, described in DE-OS No. 28 32 447, a metallic switch housing which is externally threaded and is screwed into a correspondingly tapped bore in the wheel rim defines a reference pressure chamber. That chamber is closed off on the side facing the interior of the tubeless tire by a metallic membrane against which a switch contact makes contact. The pressure in the reference pressure chamber is so set that, for a satisfactory pressure in the tire, the membrane making contact with the switch contact closes a circuit for induced electric current. When the air pressure in the tire is too low, the gas in the reference pressure chamber presses the membrane towards the interior of the tire until the electric circuit is interrupted. The circuit connected to this switch is constituted as a resonant circuit, so that the tire pressure can be monitored by an alarm circuit that is wirelessly coupled to the resonant circuit once in every revolution of the wheel.
Although in this known switch unit the gas temperature in the reference pressure chamber adjusts itself to the air temperature in the tire, it can happen that, by heat radiation from, and heat conduction through, the rim to the metallic switch housing, the temperature in the reference pressure chamber can become higher than the air temperature in the tire, as may happen in prolonged operation of the brake. In such a case, the electrical circuit is opened and a false signal is provided to the alarm circuit.